THE ATOMIC SOFTNESS MATRIX

The conventional approach to the electronegativity equalization princi ple suffers from a serious conceptual drawback that stems from discont inuities in the first derivative of the electronic energy with respect to the total charge of a molecule. A formalism that avoids the result ing ill-defined atomic hardness matrices employs a simple definition o f the atomic softness matrix sigma. The matrix sigma, which is a gener alization of the Huckel atom-atom polarizability matrix, can be easily and rigorously computed to arbitrary accuracy with any of the contemp orary electronic structure methods. Properties of the atomic softness matrix are reviewed and illustrated with several numerical examples in volving the first-row hydrides and a linear polyyne.